High reliability, long lifetime, negative ion source

ABSTRACT

A negative ion source includes a plasma chamber, a microwave source, a negative ion converter, a magnetic filter and a beam formation mechanism. The plasma chamber contains gas to be ionized. The microwave source transmits microwaves to the plasma chamber to ionize the gas into atomic species including hyperthermal neutral atoms. The negative ion converter converts the hyperthermal neutral atoms to negative ions. The magnetic filter reduces a temperature of electrons provided between the plasma chamber and the negative ion converter. The beam formation mechanism extracts the negative ions.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/844,054 filed on Jul. 9, 2013, which is hereby incorporated byreference in its entirety.

FIELD

The present application relates generally to the field of negative ionsources. More specifically, the present application relates to systemsand methods for producing hyperthermal neutral atoms and converting themto negative ions via interaction with a cesiated conversion cone andextracting the negative ions into a high voltage (˜30 kV) ion beam.

BACKGROUND INFORMATION

This section is intended to provide a background or context to theinvention recited in the claims. The description herein may includeconcepts that could be pursued, but are not necessarily ones that havebeen previously conceived or pursued. Therefore, unless otherwiseindicated herein, what is described in this section is not prior art tothe description and claims in this application and is not admitted to beprior art by inclusion in this section.

Ion sources are used across a wide range of applications including basicscience research, medical applications, and semiconductor production. Inmany cases, the performance and reliability of very large, complex, andexpensive systems is limited by the performance and reliability of theion source, which often represents a relatively small part of the totalsystem in terms of size and cost. Thus, advances in ion sourcetechnologies can lead to drastic improvements in system performancerelatively quickly. However, ion sources are complex devices that oftensuffer from reliability issues when pushed to high currents, as is oftendemanded by the rest of the system.

Lifetime and reliability issues are especially troublesome for existingnegative ion sources, such as negative hydrogen (H−) ion sources.Nonetheless, negative ion sources are still commonly used across a broadrange of applications due to the fact that for many applications,downstream system components require negative rather than positive ions.Conventional negative ion sources may have, for example, a relativelyshort lifetime of only a few hundred hours. This lifetime decreases evenfurther when operated at full power (e.g., 15 mA). Furthermore,conventional negative ion sources may encounter other problems includinghigh power requirements (15 kW) and high gas load (18-20 sccm) on thedownstream vacuum components.

A reliable, long lifetime negative ion source has applications insilicon cleaving for photovoltaic semiconductor applications, isotopeproduction and separation, cyclotron injection systems, and acceleratormass spectrometry. Cyclotrons are widely used across medical andindustrial fields. As technology continues to develop, it appears thation source injectors could become limiting factors with regard to beamcurrent and accelerator performance. There are several technical reasonswhy it is preferable to inject negative rather than positive ions intocyclotrons, and the low current and short lifetime of existing ionsources will potentially limit the performance of next-generationcyclotrons. Similarly, ion beams are used in a wide range of settings inthe semiconductor industry. Better ion sources translate to cheaper,more efficient, and more effective production techniques for circuitcomponents that are the building blocks of all modern IC-basedtechnologies

In another example, the negative ion source may be used in the field ofmagnetic confinement fusion energy. For decades scientists have soughtto develop an energy source based on nuclear fusion reactions, as itcould potentially provide an essentially unlimited amount of cleanenergy with virtually no harmful byproducts. Though fusion energytechnologies have advanced immensely over the past several decades,there are still a number of technical challenges that prevented thedevelopment of a clean fusion energy reactor. One challenge faced byfusion energy is unreliable high current negative ion sources. Existingnegative ion fusion injectors use filaments and/or magnetically coupledplasmas that suffer from many of the deficiencies discussed above. Areliable, long lifetime negative ion source could drastically increasethe ion source conversion efficiency, lifetime, reliability, and currentoutput. Developing such a negative ion source could be a major stepforward in developing a clean, reliable fusion energy source.

A need exists for improved technology, including technology related to anew type of ion source that can produce high DC current output (up to 10mA) and have a long lifetime (greater than 1 month).

SUMMARY

An exemplary embodiment relates to a pulsed or continuous wave negativeion source including a plasma chamber, a microwave source, a negativeion source converter, a magnetic filter and a beam formation mechanism.The plasma chamber contains a gas to be ionized. The microwave sourcetransmits microwaves to the plasma chamber to ionize the gas into atomicspecies including hyperthermal neutral atoms. The negative ion sourceconverter converts the hyperthermal neutral atoms to negative ions. Themagnetic filter reduces the temperature of electrons between the plasmachamber and the negative ion source converter. The beam formationmechanism extracts the negative ions.

Another embodiment relates to a continuous wave negative ion source thatincludes a plasma chamber, a microwave source, a negative ion converter,a magnetic filter and a beam formation mechanism. The plasma chambercontains gas to be ionized. The microwave source transmits microwaves tothe plasma chamber to ionize the gas into atomic species includinghyperthermal neutral atoms. The negative ion converter converts thehyperthermal neutral atoms to negative ions. The magnetic filter reducesthe temperature of electrons between the plasma chamber and the negativeion converter. The beam formation mechanism extracts the negative ions.

Yet another embodiment relates to a method of producing negative ions.The method includes providing a gas to be ionized in a plasma chamber,transmitting microwaves from a microwave source to the plasma chamber toionize the gas such that hyperthermal neutral atoms of the gas areproduced, converting the hyperthermal neutral atoms to negative ions viaan interaction with a negative ion source converter, and extracting thenegative ions with a beam formation mechanism.

Additional features, advantages, and embodiments of the presentdisclosure may be set forth from consideration of the following detaileddescription, drawings, and claims. Moreover, it is to be understood thatboth the foregoing summary of the present disclosure and the followingdetailed description are exemplary and intended to provide furtherexplanation without further limiting the scope of the present disclosureclaimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide furtherunderstanding of the invention, are incorporated in and constitute apart of this specification, illustrate embodiments of the presentdisclosure and together with the detailed description serve to explainthe principles of the present disclosure. No attempt is made to showstructural details of the present disclosure in more detail than may benecessary for a fundamental understanding of the present disclosure andthe various ways in which it may be practiced.

FIG. 1 is a top view of an exemplary embodiment of a negative ionsource.

FIG. 2 is schematic top view of the negative ion source of FIG. 1.

FIG. 3 is a cross-sectional view of the negative ion source of FIG. 1through the line G-G.

FIG. 4 is a schematic cross-sectional view of the negative ion source ofFIG. 1 through the line G-G.

FIG. 5 is an isometric cross-sectional view of a waveguide break of awaveguide of the negative ion source of FIG. 1.

FIG. 6 is an isometric view of the waveguide of FIG. 5.

FIG. 7 is a top view of the waveguide of FIG. 5.

FIG. 8 is a side view of the waveguide of FIG. 5.

FIG. 9 is a schematic side view of a microwave source of the negativeion source of FIG. 1.

FIG. 10 is a schematic front view of a microwave source of the negativeion source of FIG. 1.

FIG. 11 is a schematic top view of a microwave source of the negativeion source of FIG. 1.

FIG. 12 is a schematic isometric view of a microwave source of thenegative ion source of FIG. 1.

FIG. 13 is a front view of a negative ion converter of the negative ionsource of FIG. 1.

FIG. 14 is another front view of a negative ion converter of thenegative ion source of FIG. 1.

FIG. 15 is a schematic illustration of the negative ion source of FIG.1.

FIG. 16 is another schematic illustration of the negative ion source ofFIG. 1.

FIG. 17 is another schematic illustration of the negative ion source ofFIG. 1.

FIG. 18 is a photograph of the positive ion source of FIG. 1.

FIG. 19 is a graph illustrating negative yield of a cesiated molybdenumsurface under bombardment of neutral atoms.

FIG. 20 is a potential energy diagram for the H₂ molecule.

FIG. 21 illustrates electron impact H₂ molecular dissociation crosssections.

FIG. 22 illustrates calculated dissociation rates of H₂ molecules viaelectron collisions with energies from Maxwellian distributionscharacterized by an electron temperature.

FIG. 23 illustrates a cross section for electron detachment ine+negative collisions.

FIG. 24 illustrates Langmuir probe measurements for the two-chambermulticusp negative ion source.

FIG. 25 illustrates 1993 results from Lee. Shown are measured negativecurrent densities as a function of gas flow rate using a Lisitano coilion source and a Cs converter.

FIG. 26 illustrates resonant charge-exchange cross section for theH^(o)+H⁻ system.

DETAILED DESCRIPTION

Before turning to the figures, which illustrate the exemplaryembodiments in detail, it should be understood that the presentdisclosure is not limited to the details or methodology set forth in thedescription or illustrated in the figures. It should also be understoodthat the terminology is for the purpose of description only and shouldnot be regarded as limiting. An effort has been made to use the same orlike reference numbers throughout the drawings to refer to the same orlike parts.

Recent advances in positive ion sources using the resonant interactionof 2.45 GHz microwaves in gas in the presence of an 875 Gauss magneticfield have led to intense DC positive ion beams. Twenty five percentefficiency in converting gas into positive ions using 1 kW of microwavepower has been routinely observed. Microwave ion sources are becomingincreasingly common in commercial use due their inherent DC operatingcapability.

Referring generally to the figures, an exemplary embodiment relates to anegative ion source 100 capable of reliably operating above 10 mA forseveral months at a time using a mechanism to produce hyperthermal atoms(2-5 eV) via an interaction of a cesiated surface, and volume negativeion production techniques with a neutral atom beam generated by amicrowave ion source that utilizes an electron cyclotron resonance at2.45 GHz. The CW negative ion source 100 is a high current, highreliability, long lifetime negative ion source.

In order to produce such a negative ion source, surface production ofnegative ions is built upon by scattering hyperthermal atoms from acesiated low work function surface. Hyperthermal atomic energy refers toatom energies greater than 2 eV. The negative yield from atomichydrogen)(H^(o) incident on a cesiated molybdenum surface is shown inFIG. 23. The four points are measurements, and the solid line istheoretical prediction. The hyperthermal contribution would occur in therange (1/kT)<1.

A significant advancement that has occurred since the Stevens Institutework described in Brian S. Lee and M. Seidl, Appl. Phys. Lett. 61 (24),2857 (1992) (hereafter “Lee”), the entire contents of which isincorporated by reference herein, is the development of practical andproven 2.45 GHz microwave H⁺ sources (MWS). This development isdescribed, for example, in Terence Taylor and John S. C. Wills, NuclearInstruments and Methods in Physics Research A309, 37 (1991) (hereafter“Taylor”), the entire contents of which is incorporated by referenceherein. The MWS high gas and power efficiencies suggest that the plasmagenerator should also provide efficient production of neutral atombeams. The 2.45 GHz MWS plasma generator has demonstrated CW modeoperation at many laboratories for many different industrialapplications. Other neutral atom generators that have previously beeninvestigated for this purpose are the multicusp source, the arcdischarge source, and the 2.45 GHz Lisitano-Coil source.

Negative Beam Current and Emittance Considerations

For a circular aperture, a relationship between a normalized rmsemittance (ε_(rms,n)) and the extracted negative ion temperature isdescribed as follows:

ε_(rms,n)=(r/2)√{square root over (kT/(mc ²))}  (1)

where r is the radius of the emission aperture, kT is the plasma iontemperature, and mc² is the rest mass of the negative ion.

A concept related to emittance is beam brightness, B. This may bedefined as B=I/(ε_(x)ε_(y)) where ε_(x) and ε_(y) are the transversebeam emittances. For purposes of this disclosure, where an axisymmetricsource is expected, the definition of B becomes

B=I/ε _(r) ²  (2)

where ε_(r) is the beam emittance in (r,r′) space. In one embodiment,the desired current is I=10 to 15 mA.

The negative ion current density, j⁻, is defined by the followingequation:

j ⁻ =I/(r ²).  (3)

If one solves equation (3) for r, substitutes that relationship intoequation (1), and then uses that emittance expression in equation (2),one arrives at the following expression for beam brightness:

B=j/(kT).  (4)

According to equation (4), a brighter ion source is directly related tothe extracted current density j, and inversely related to the iontemperature, kT.

Using the equations described above, in theoretical Example 1, an ionemission aperture has a radius r=0.4 cm and I=10 mA. Equation (3) givesj⁻=20 mA/cm². Considering 500 mA/cm² atomic hyperthermal hydrogendensity was measured in Lee, it is very likely that at least 4% of thisatomic hydrogen flux could be converted to negative ions. Furthermore,substituting several eV ion temperature into equation (1) gives anormalized rms emittance considerably lower than a radio frequencyquadrupole (RFQ) input beam design emittance of 0.25 (πmm-mrad), rmsnormalized (ε_(rms,n)) Thus, from the parallel negative ion energymeasurements reported in [1], a few eV negative ion temperature isexpected.

The following sections provide details on coupling a modern 2.45 GHzmicrowave plasma source to a cesiated converter surface. Then, an ionextraction system is proposed which simultaneously accelerates thenegative ion beam to 35 keV and separates the parasitic electroncomponent. The goal is to achieve the beam quality described above witha long run time (greater than 6 months) negative ion source.

Hyperthermal Neutral Hydrogen Atom Generation

Referring to the figures more particularly, as illustrated in FIG. 1, anexemplary embodiment of a system for producing a CW negative ion source100 includes a microwave source 110, an ion source plasma chamber 120, awaveguide 130, a magnetic filter 140, a negative ion converter 150 and anegative ion beam formation mechanism 160.

In one embodiment, the microwave source 110 is a 2.45 GHz microwavesource. The microwave source 110 is configured to transmit microwavesthat ionize gas provided in the ion source plasma chamber 120 to convertthe molecular gas into atomic species, in particular, hyperthermalneutral atoms. The microwave source 110 works on an electron cyclotronresonance (ECR) principle, thus requiring an ˜875 G on-axis magneticfield. FIG. 12 illustrates a positive deuterium ion source.

The MWS has consistently demonstrated high gas efficiency and highproton fraction (˜90%). The gas efficiency h is defined as the fractionof proton or deuteron gas nuclei (in the form of the molecular gas)converted to charged particle beam. In practical units, h is given by

h=6.95×I(A)/Q(sccm).  (5)

A LEDA proton accelerator at Los Alamos is described in Joseph D.Sherman, et. al., Review of Scientific Instruments 73(2), 917(2002), theentire contents of which is incorporated by reference herein. The LEDAproton accelerator produced 154 mA of hydrogen ions at 4.1 sccm gas flowat 90% proton fraction. This gives h_(Hn+)=0.26. In comparison, thedeuteron ion source of the present application has produced up to 53 mAat 45 keV beam energy with 1.9 sccm gas flow. This gives a deuteron ionproduction efficiency h_(Dn+)=0.19.

Theoretically, dissociation of molecules to neutral atoms followed byionization of the neutral atoms to positive ions is more likely thanionization of molecules to negative molecular ions followed bydisassociation of negative molecular ions to positive ions. Thus, a goodatomic ion source should also be a good neutral atom beam source. The150 mA beam current observed in the LEDA MWS corresponds to 0.9×10¹⁸charged particles/sec (p/s). At 4.1 sccm H₂ gas flow, the number ofneutral hydrogen atoms entering the source is 3.7×10¹⁸ p/s. Correctingthis latter number for charged particles leaving the source results in2.8×10¹⁸ neutral hydrogen atoms/s leaving the source. The emissionaperture radius for the LEDA ion source is 0.43 cm, which gives an areaof 0.58 cm². If all the neutral particles leaving the LEDA MWS are inthe form of dissociated H₂ molecules, the H^(o) electrical equivalentcurrent density is 747 mA/cm². Therefore, to function as a 10 mA/cm²negative ion source, a 1.3% conversion efficiency from neutral atoms tonegative ions would be required.

Hyperthermal H^(o) (energy >2 eV) may be generated by a high temperatureelectron plasma through the Franck-Condon dissociation mechanism of theH₂ molecule described in Brian S. Lee, “Surface Production of negativeIons by Backscattering Hyperthermal Hydrogen Atoms”, Ph.D. Thesis,Department of Physics and Engineering, Stevens Institute of Technology,Castle Point Station, Hoboken, N.J. (1993) (hereafter “Brian S. Lee”),the entire contents of which is incorporated by reference herein. TheFranck-Condon region in the potential energy curves for electronicstates up to 30 eV for the H₂ and H₂ molecules are shown in FIG. 20.Onset and minimum separation energies for dissociated H^(o) and H⁺ areshown. Dissociation cross sections for the H₂ molecule are shown in FIG.21. Both FIGS. 20 and 21 indicate an electron energy threshold of about8.8 eV for direct dissociation of the H₂ molecule. The reaction rates<sv> calculated from the dissociation cross sections and electronvelocities (based on Maxwellian distributions with temperature kT_(e))are shown in FIG. 22. The dissociation reaction rates tend to saturatearound 8 eV electron temperature.

Taylor reported that double cylindrical Langmuir probe measurementstypically gave 20 eV electron temperatures. This high temperature putsthe H₂ dissociation reaction rate into the saturated portion of FIG. 22.This experimental data combined with the theory presented above suggeststhat molecular dissociation rates will be high in the negative ionsource of the present application.

Eqns. (4)-(5) give the continuity equation for the production and lossrates of H^(o):

n _(e) n(H ₂)<sv _(e) >V=n _(Ho) v _(Ho) A/(4a)  (6)

where

-   -   n_(e)=electron density in LEDA MWS    -   n(H₂)=density of H₂ in the LEDA MWS    -   V=LEDA ion source plasma volume    -   A=LEDA ion source plasma surface area    -   a=destruction probability of H^(o) on bounce from wall    -   1=V/A=LEDA ion source length        By invoking plasma charge neutrality (n₊=n_(e)), solving for n₊        from the positive current density extracted from j₊=n₊v₊=0.26        A/cm², and taking v₊ from a reasonable plasma ion temperature of        1 eV, the following Eqn. (7) is obtained:

n _(e)=1.2×10¹² e/cc  (7)

The density of H₂ in the LEDA MWS (n(H₂)) is found by assuming molecularH₂ flow through a 0.86 cm diameter emission aperture, calculating sourcepressure (T) at 4 sccm H₂ gas flow, and converting the source pressureto number density through the use of Loschmidt's number. The result is

n(H ₂)=7.1×10¹³ H ₂ /cc  (8)

Solving for the H^(o) flux (_(o)) from eqn. (6) and substituting theLEDA ion source parameters and maximum <sv_(e)>=7.8×10⁻⁹ cc/s from FIG.14, one finds

F _(Ho) =n _(Ho) V _(Ho)/4=an _(e)(n(H ₂)<sv _(e) >l=6.6×10¹⁸ H ^(o)/(cm² −s)  (9)

where a=1 is assumed. Converting F_(Ho) to a charge equivalent, one ofordinary skill in the art calculates 1.056 A/cm². In the discussionabove, it was found that the highest H^(o) flux would be 0.75 A/cm².This analysis suggests that the 2.45 GHz MWS in TE₁₀₀ mode is likely anexcellent source of hyperthermal H^(o). FIG. 16 shows a conceptualanimation of the principal components in the proposed 2.45 GHz H^(o)source.

It is desirable to have the plasma chamber 120 at high voltage and themicrowave source 110 at ground potential. Thus, the plasma chamber 120and microwave source 110 must be electrically isolated. This isaccomplished by transmitted the microwaves generated by the microwavesource 110 into the plasma chamber 120 via a waveguide 130.

In one embodiment, the waveguide 130 is generally disk-shaped, includinga flange 131 and a waveguide break 132 in the center of the waveguide130. The waveguide break 132 is configured to be insulated with air. Aplurality of waveguides 130 may be stacked (see Fig. X) or a singlewaveguide 130 may be used to deliver the microwave power from themicrowave source 110 to the plasma chamber 120. The waveguide 130 may berigid at one end 130A of the waveguide break 132, while flexible at theother end 130B of the waveguide break 132 (i.e., the end closest to theplasma chamber 120) in order to facilitate installation and maintenanceof the waveguide.

The waveguide 130 may include commercially available components such asa directional coupler 133, an autotuner 134 and a circulator 135. Thedirectional coupler 133 is configured to detect the phase and amplitudeof a microwave to determine both its forward and reflected power. Theautotuner 134 is configured to match the load impedance (i.e., theimpedance of the plasma chamber 120) to that of the source (i.e., themicrowave source 110), thereby reducing reflected power and maximizingthe coupling power to the plasma chamber 120. In order to match theimpedances between the microwave source 110 and the plasma chamber 120,stubs 136 may be inserted at different lengths and various depths alongthe waveguide 130 based on instructions generated by the autotuner 134.The circulator 135 is three-port device configured to selectively directmicrowaves to a specific port based on the direction of wavepropagation. In order to protect the microwave source 110 from reflectedmicrowave energy and to increase the efficiency of microwave generation,the circulator 135 may include a “dummy” load configured to absorb thereverse power.

Negative Ion Conversion

The negative ion converter 150 is configured to direct an atomic beamonto a cesiated surface 151, for example, a cesiated molybdenum plate,where the atomic beam is converted to negative ions via cesiumcatalysis. The surface of the negative ion converter 150 is at anegative potential such that the negative ions are accelerated anddeflected by an ambient 875 Gauss magnetic field. The negative ionconverter 150 is located adjacent to an emission hole appropriate fornegative ion extraction in order to form a low energy negative ion beam.Ion beam diagnostics may be provided to monitor neutral particle energy(calorimetric), neutral particle flux, and electron temperature anddensity near the extractor.

The negative ion beam is formed in a reduced plasma density environment.The neutral atoms will leave the 2.45 GHz plasma chamber accompanied byother charged particles including hot and cold electrons, as well aspositive ions. It is important to reduce the high temperature electrondensity in the region of the negative ion converter 150. If thetemperature is not reduced, for example, the efficiency of the negativeion source is decreased. It is well known negative ion destruction crosssection (see FIG. 17) is the impact of negative ions by electrons withenergy >2 eV. The rapid decrease in the cross section at electronenergies less than 1 eV arises because the H^(o) electron affinity is0.75 eV. To prevent this occurrence, the plasma chamber 120 and thenegative ion converter 150 are separated by a magnetic filter 140. Inone embodiment, the magnetic filter 140 may be a tunable magnetic dipolefield. The magnetic filter 140 is provided circumferentially around thejunction between the plasma chamber 120 and the negative ion converter150.

As described in K. N. Leung, K. W. Ehlers, and M. Bacal, “Extraction ofVolume Produced negative Ions from a Multicusp Sources”, Rev. Sci.Instrum. 54(1), 56 (January, 1983) (hereafter “Leung”), the entirecontents of which is incorporated by reference herein, a magneticfiltering technique using a magnetic filter 140 between the H^(o)production chamber and the negative extraction region has been shown toreduce electron temperatures in the extraction region. FIG. 24 showsLangmuir probe measurements from Leung taken in a magnetically-filteredcusp field negative source. The top portion of the figure shows probetraces in the source chamber where an electron temperature of 1.4 eV ismeasured. The lower portion of FIG. 24 shows the probe traces in theextraction chamber, and the electron temperature there has been reducedto 0.35 eV. Considering the very high electron temperatures observed inthe microwave source, a magnetic filter will be required to reduce theelectron temperature.

By addressing challenges including high negative ion temperature anddifficulty reducing the high temperature electron density at theconverter and extractor regions, the system will produce a long-lived,high current density, DC negative ion source. Furthermore, negative ionsource may decrease power requirements by 85% and downstream gas load by80% relative to existing high current negative ion sources.

FIG. 19 shows a schematic layout of the negative ion production andextraction region. The neutral atoms progress from right to left, movingfrom the plasma chamber 120 (i.e., the neutral atom generator) throughthe dipole magnetic filter 140. Fast electrons are removed by the filter140 from the source particle flux. Slow electrons and positive ions passthrough the filter 140 by mechanisms discussed in Leung. The neutralatoms pass through the filter field, and some neutral flux strikes thenegative ion converter 150 (i.e., the cesiated molybdenum surfaceconverter). This leads to negative ion formation with yield determinedby neutral atom conversion efficiency (as shown in FIG. 19). Thenegative ion converter 150, may be for example, a long cone (see FIG.19) to maximize a surface area for negative ion production and toprovide as large a solid angle as possible for capturing thesurface-produced negative ions.

The cesiated molybdenum surface should be heated before the firstapplication of the cesium metal from a cesium dispenser. It may benecessary to continuously feed Cs to maintain the low work functionsurface for optimum negative ion yield. The converter surface shouldalso be isolated from ground, as imposing some small negative voltage onthe converter may enhance the negative ion yield.

The total negative ion yield <Y(kT)> shown in FIG. 11 is plotted vs.(1/kT). This quantity is obtained from an integral of a Maxwellianneutral atom energy distribution over a negative ion yield as a functionof the perpendicular energy of the back scattered atom. The points inFIG. 11, located in the (1/kT) range of 4-6 eV, are taken fromthermal-atom yield measurements. The solid line is a theoretical resultfor predicted electron transfer to reflected neutral atoms from a metalsurface.

Langmuir probe measurements will be made after the magnetic filter inthe converter region of the ion source. A filter modified from that usedin Leung will most likely be required because of the high electrontemperature of the microwave source. A continuously variableelectromagnet may be preferred.

Negative Ion Extraction

Negative ions are extracted via the negative ion beam formationmechanism 160 (see FIG. 20). Negative ion extraction from the negativeion converter 150 takes place in a magnetic field-free region, soelectrons follow the negative ion trajectory. One of ordinary skill inthe art will expect an e: negative ion ratio >1, so some care has to betaken in dumping the accelerated electron power. The negative ion andelectron currents are separated after the beam reaches full energy byimposition of a weak magnetic field. The ion source main body is thenoffset from the accelerator beam line by the bend angle of the negativeion beam. A suitable water-cooled beam dump is installed at the electrondump location. The PBGUNS simulation code, as described in Jack Boers,Proc. Of the 1995 Particle Accelerator Conference, IEEE Catalog Number95CH35843, p. 2312; R. F. Welton, et. al., Rev. of Sci. Instrum. 73 (2),1013 (2002), the entire contents of which are incorporated by referenceherein, is used to design, simulate, and verify an optimized negativeion extraction system.

An emission aperture in the negative ion beam formation mechanism 160(left side of FIG. 19) will have to be optimized for negative ion beamproduction, emittance, and beam brightness requirements summarized ineqns. (1) and (4).] Results found at the Stevens Institute (see Brian S.Lee) are shown in FIG. 25. The maximum measured negative ion currentdensity reported was 0.25 mA/cm² with ˜420 W of microwave power usingthe Lisitano coil ion source (see Lee and Brian S. Lee). If a 6.0 cmlong cesiated cone surface converter with a neutral atom openingdiameter of 4.0 cm narrowing down to a 0.8 cm diameter emission apertureis used, then the converter surface area is 47 cm². The observed 0.25mA/cm² current density combined with this surface area gives 12 mA ofnegative ions. Furthermore, the ion source will operate with at leastthree times greater microwave power —at least 1200 W. Also, thecone-shaped converter 150 surface may recycle the negative ions that donot pass through the emission aperture. This recirculation may beespecially effective for the interaction of slow negative ions from theconverter with slow neutral atoms from the generator that are directedtoward the plasma electrode aperture. This situation will bring intoplay the large resonant charge exchange of negative ions interactingwith neutral atoms, shown in FIG. 26. Another favorable feature of themicrowave source is the conversion of all molecular gas to atomicspecies. Even if a large majority of the neutral atoms escape the plasmachamber below hyperthermal energies, the very large neutral atom fluxplus the recirculation of negative ions near the plasma aperture willlead to enhanced negative ion production.

Diagnostic Procedures for Checking Performance of the Neutral AtomGenerator

The neutral atom velocity and flux are critical for the subsequentnegative ion creation. A rotating disk assembly 170 with slots 171offset by an angle f_(a) may be provided as a diagnostic tool to monitorthe neutral atom velocity. The rotating disk assemblies 170 willseparated by a known distance L. By requiring the neutral atom time offlight between the disks 170 to equal the rotation time for the secondslot 171 to rotate to the beam pulse location, one of ordinary skill inthe art derives the equation:

w=bf _(a) c/L  (10)

where w is rotation velocity in rad/s, b is relativistic velocity ofneutral atoms, and c is the velocity of light. Converting this equationto rpm, taking b to be the velocity of 2 eV neutral atoms, L=25 cm, andf_(a)=2°, one finds a rotational velocity of 25,000 rpm. The detectorfor transmitted neutral atoms may be a negatively biased metal wire, sothat the secondary electrons produced by neutral atomic interactionswith the metal ribbon would be detected.

Other methods for determining neutral particle velocities have beenused. One method, as described in Bernardo Jaduszliwer and Yat C. Chan,“Atomic Velocity Distributions Out of Hydrogen Maser Dissociators”,Chemistry and Physics Laboratory, The Aerospace Corp., P.O. Box 92957,Los Angeles, Calif. 90009, the entire contents of which is incorporatedherein, uses the magnetic moment of neutral atoms interacting with anon-uniform magnetic field. That work used an RF dissociator ofmolecules and would be sensitive to hyperthermal neutral atoms. Thatwork led to peak atom energies of about 0.06 eV. The degradation inenergy from the ˜2 eV expected energy from the molecular dissociationwas attributed to inelastic processes. The electrical polarizability ofalkalai clusters [12] deflected by two-wire electrical fields andchoppers was used to gain velocity information.

Optical spectroscopic methods have been used to study anomalous Dopplerbroadening of neutral atom lines, as described in Zoran Petrovic andVladimir Stojanovic, “Anomalous Doppler Broadening of Hydrogen Lines Dueto Excitation by Fast Neutrals in Low Pressure Townsend Discharges”,Mem. S. A. It. Vol 7, 172 (2005); K Akhtar, John Scharer, and R. I.Mills, “Substantial Doppler Broadening of Atomic Hydrogen Lines in DCand Capacitively Coupled RF Plasmas”, J. Phys. D: Appl. Phys. 42,135207(2009)., the entire contents of which is incorporated by referenceherein. Low cost spectrometers are available which can be used tomeasure the atomic Balmer lines. Optical spectroscopic methods may bethe easiest diagnostic to implement, and they may give the desiredinformation on the hyperthermal vs. thermal neutral atom distribution.Careful consideration will be given to this diagnostic, as it may be thesimplest and least expensive diagnostic to install for neutral atomenergy and density measurements.

Obtaining neutral atom divergence and beam energy models are discussedin E. C. Samano, W. E. Carr, M. Seidl, and Brian S. Lee, Rev. of Sci.Instrum. 64(10), 2746 (October, 1993), the entire contents of which areincorporated by reference herein. The atomic beam divergence can bededuced by observing the burn pattern of neutral atoms in a MoO₃ film.Reduction of neutral atoms changes the MoO₃ film (yellow-green) to MoO₂(blue). calorimetric measurements with different calorimeter materialswill be used to deduce temperature increases associated with the neutralatom beam. Even for hyperthermal neutral atoms, expected power to thecalorimeter may be 1 W or less, so special care will need to beexercised in calorimeter design.

As utilized herein, the terms “approximately,” “about,” “substantially”,and similar terms are intended to have a broad meaning in harmony withthe common and accepted usage by those of ordinary skill in the art towhich the subject matter of this disclosure pertains. It should beunderstood by those of skill in the art who review this disclosure thatthese terms are intended to allow a description of certain featuresdescribed and claimed without restricting the scope of these features tothe precise numerical ranges provided. Accordingly, these terms shouldbe interpreted as indicating that insubstantial or inconsequentialmodifications or alterations of the subject matter described and claimedare considered to be within the scope of the invention as recited in theappended claims.

It should be noted that the term “exemplary” as used herein to describevarious embodiments is intended to indicate that such embodiments arepossible examples, representations, and/or illustrations of possibleembodiments (and such term is not intended to connote that suchembodiments are necessarily extraordinary or superlative examples).

The terms “coupled,” “connected,” and the like as used herein mean thejoining of two members directly or indirectly to one another. Suchjoining may be stationary (e.g., permanent) or moveable (e.g., removableor releasable). Such joining may be achieved with the two members or thetwo members and any additional intermediate members being integrallyformed as a single unitary body with one another or with the two membersor the two members and any additional intermediate members beingattached to one another.

References herein to the positions of elements (e.g., “top,” “bottom,”“above,” “below,” etc.) are merely used to describe the orientation ofvarious elements in the FIGURES. It should be noted that the orientationof various elements may differ according to other exemplary embodiments,and that such variations are intended to be encompassed by the presentdisclosure.

It is important to note that the construction and arrangement of thelong lifetime, high current, continuous wave (CW) negative ion sourceshown and/or described in the various exemplary embodiments isillustrative only. Although only a few embodiments have been describedin detail in this disclosure, those skilled in the art who review thisdisclosure will readily appreciate that many modifications are possible(e.g., variations in sizes, dimensions, structures, shapes andproportions of the various elements, values of parameters, mountingarrangements, use of materials, colors, orientations, etc.) withoutmaterially departing from the novel teachings and advantages of thesubject matter described herein. For example, elements shown asintegrally formed may be constructed of multiple parts or elements, theposition of elements may be reversed or otherwise varied, and the natureor number of discrete elements or positions may be altered or varied.The order or sequence of any process or method steps may be varied orre-sequenced according to alternative embodiments. Other substitutions,modifications, changes and omissions may also be made in the design,operating conditions and arrangement of the various exemplaryembodiments without departing from the scope of the present invention.

1. A negative ion source comprising: a plasma chamber containing a gasto be ionized; a microwave source configured to transmit microwaves tothe plasma chamber to ionize the gas into atomic species includinghyperthermal neutral atoms; a negative ion source converter configuredto convert the hyperthermal neutral atoms to negative ions; a magneticfilter configured to reduce a temperature of electrons between theplasma chamber and the negative ion source converter; and a beamformation mechanism configured to extract the negative ions.
 2. Thenegative ion source of claim 1, wherein the negative ion source is acontinuous wave negative ion source.
 3. The negative ion source of claim1, wherein the plasma chamber has a high voltage and the microwavesource is at ground potential.
 4. The negative ion source of claim 1,further comprising at least one waveguide configured to electricallyisolate the plasma chamber and the microwave source, the at least onewaveguide comprising a flange configured to allow a break in thewaveguide to be insulated with air.
 5. (canceled)
 6. The negative ionsource of claim 1, wherein the gas to be ionized is hydrogen, thehyperthermal neutral atoms produced are hyperthermal neutral hydrogenatoms, and the negative ions produced are negative hydrogen ions.
 7. Thenegative ion source of claim 1, wherein the microwave source comprises a2.45 GHz microwave source.
 8. The negative ion source of claim 1,wherein the negative ion source converter comprises a cone-shapedcesiated surface.
 9. The negative ion source of claim 1, wherein themagnetic filter comprises a tunable magnetic dipole field providedcircumferentially around a junction between the plasma chamber and thenegative ion converter.
 10. The negative ion source of claim 1, whereinthe beam formation mechanism extracts the negative ions at a voltage of30 kV.
 11. A negative ion source comprising: a plasma chamber containinggas to be ionized; a microwave source configured to transmit microwavesto the plasma chamber to ionize the gas into atomic species includinghyperthermal neutral atoms; a negative ion converter configured toconvert the hyperthermal neutral atoms to negative ions; and a beamformation mechanism configured to extract the negative ions.
 12. Thenegative ion source of claim 11, wherein the plasma chamber has a highvoltage and the microwave source is at ground potential.
 13. Thenegative ion source of claim 11, further comprising at least onewaveguide configured to electrically isolate the plasma chamber and themicrowave source, the at least one waveguide comprising a flangeconfigured to allow a break in the waveguide to be insulated with air.14. (canceled)
 15. The negative ion source of claim 11, wherein themicrowave source comprises a 2.45 GHz microwave source.
 16. The negativeion source of claim 11, wherein the negative ion converter comprises acone-shaped cesiated surface.
 17. (canceled)
 18. The negative ion sourceof claim 1, wherein the beam formation mechanism extracts the negativeions at a voltage of 30 kV.
 19. A method of producing negative ions, themethod comprising: providing a gas to be ionized in a plasma chamber;transmitting microwaves from a microwave source to the plasma chamber toionize the gas such that hyperthermal neutral atoms of the gas areproduced; converting the hyperthermal neutral atoms to negative ions viaan interaction with a negative ion source converter; and extracting thenegative ions with a beam formation mechanism.
 20. The method of claim19, further comprising reducing a temperature of electrons providedbetween the plasma chamber and the negative ion source converter with amagnetic filter.
 21. The method of claim 19, further comprisingelectrically isolating the plasma chamber and the microwave source withat least one waveguide comprising a flange configured to allow a breakin the waveguide to be insulated with air.
 22. The negative ion sourceof claim 1, wherein the negative ion source converter comprises acone-shaped surface.
 23. The negative ion source of claim 1, wherein thenegative ion source converter comprises a cesiated surface.